1. Field of the Invention
The invention relates to a collector unit for illumination systems with a wavelength of ≦193 nm, preferably ≦126 nm, especially preferably with wavelengths in the EUV region with at least one mirror shell which receives the rays of a beam bundle coming from an object and has an optical effect with regarding the rays of the beam bundle.
Preferably, the rays of the beam bundle impinges under an angle of <20° to the surface tangent of the mirror shell.
Moreover, the invention also provides an illumination system with such a collector, a projection exposure system with an illumination system in accordance with the invention as well as a method for exposing microstructure.
2. Description of the Related Art
Nested collectors for wavelengths ≦193 nm, especially wavelengths in the region of X-rays, are known from a large number of publications.
U.S. Pat. No. 5,768,339 discloses a collimator for X-rays, with the collimator having several nested paraboloidal reflectors. The collimator according to U.S. Pat. No. 5,768,339 is used for the purpose of shaping an isotropically radiated ray beam of an X-ray light source into a parallel beam.
U.S. Pat. No. 1,865,441 discloses a nested collector for X-ray beams. The nested collector in the case of U.S. Pat. No. 5,768,339 is used for collimating isotropic X-rays emitted from a source into a parallel ray beam.
U.S. Pat. No. 5,763,930 shows a nested collector for a pinch plasma light source which is used to collect the rays emitted by a light source and to focus the same in a waveguide.
U.S. Pat. No. 5,745,547 shows several arrangements of multi-channel optics which are used to focus the radiation of a source, especially X-rays, by multiple reflections.
In order to achieve an especially high transmission efficiency, the invention pursuant to U.S. Pat. No. 5,745,547 proposes elliptically shaped reflectors.
DE 30 01 059 C2 discloses an arrangement for use in X-ray lithography systems which comprises nested parabolic mirrors. The nested parabolic mirrors are arranged between X-ray source and mask. These mirrors are arranged in such a way that a diverging X-ray beam bundle impinging onto the nested collector is shaped into an X-ray beam bundle emerging from the nested collector in parallel.
The arrangement according to DE 30 01 059 is merely used for achieving a favourable collimation for X-ray lithography.
The arrangement of nested reflectors as known from WO 99/27542 is used in an X-ray proximity lithography system to refocus light of a light source, so that a virtual light source is formed. The nested shells can have an ellipsoid shape.
A nested reflector for high-energy photon sources is known from U.S. Pat. No. 6,064,072 which is used for forming the diverging X-rays into a ray beam extending in parallel.
WO 00/63922 shows a nested collector which is used for collimating a neutron beam.
WO 01/08162 discloses a nested collector for X-rays which is characterized by a surface roughness of the inner, reflective surfaces of the individual mirror shells of less than 12 Å. The collectors as shown in WO 01/08162 also comprise systems with multiple reflections, especially also Wolter systems, and are characterized by a high resolution as is required for X-ray lithography for example. A further problem in illumination systems for wavelengths ≦100 nm in addition to the collection of the radiation emitted by the light source is that the light sources of such illumination systems also emit radiation of a wavelength which may lead to an undesirable exposure of the light-sensitive object in the wafer plane of the projection exposure system and optical components of the exposure system such as multilayer mirrors are heated in an impermissible way by such radiation and will rapidly degrade. Transmission filters made of zirconium can be used for filtering out such undesirable radiation. Such filters furthermore have the disadvantage of high losses of light. Moreover, they can be destroyed very easily by thermal loads. A further problem of illumination optics for EUV lithography is that the losses of light will grow strongly with the number of optical components.